1. Field of the Invention
The present invention relates to a prothesis of the upper extremity of the humerus, which prosthesis is of the type that comprises a stem anchored in the humeral canal and which is provided on its upper part with a bearing surface against which rests a hemispherical cap suitable to interact with the shoulder socket. The invention relates also to a set of humeral stems for such a prosthesis.
2. History of the Related Art
Well known are prostheses of this type that comprise a humeral stem that on its upper part presents a bearing surface provided with a clamping device for the fastening of a hemispherical cap. The humeral stem has only one bearing surface suitable to accommodate the cap. Under these conditions, the cap is at a fixed tilt angle and it does not cover in a satisfactory manner the cut plane of the extremity of the humerus so that it becomes necessary to adapt the bone to the prosthesis with all the anatomic consequences it entails.
Furthermore, in patent application EP-A1-0 549 480, a prothesis is disclosed for the upper extremity of the humerus that enables:
an effective positioning and anchoring in the humeral canal;
the recovery of the entire height of the reconstituted humerus;
a diameter of the humeral cap suitable to ensure a satisfactory covering of the cut plane of the humerus;
a satisfactory tilt angle of the humeral cap;
an appropriate retro-torsion angle of the humeral cap;
a lateral offset of the cap with respect to the humeral axis;
an anteroposterior positioning of the cap with respect to the humeral axis; and
an appropriate height of the humeral cap.
Thus, the prosthesis disclosed in patent application EP-A1-0 549 480 comprises a stem that is anchored in the humeral canal, one hemispheric cap suitable to interact with the shoulder socket, and a wedge-shaped cross piece suitable to adjust the distance from the base of the cap with respect to the bearing surface of the stem. The cross piece also allows the direction of the base of the cap to be changed with respect to the plane including the stem""s bearing surface by changing the angle determined by its end surfaces. Moreover, this prosthesis comprises means to adjust the angular position of the cap around an eccentric geometric axis with respect to its own geometric axis.
This type of prosthesis has certain drawbacks as regards the disposition of several independent elements that the surgeon must fit together when operating, in order to obtain a perfect repair of the shoulder articulation. This specific disposition is quite complex. It requires long and difficult manipulations by part of the surgeon to obtain the exact positioning of the hemispheric cap with respect to the osseous cut.
The prosthesis of the upper extremity in accordance with the invention has as an objective to provide the surgeon with the possibility of cutting the neck of the humerus according to an anatomic line clearly separating the articular portion from the osseous metaphysis.
With this in mind, the invention relates to a prosthesis of the upper extremity of the humerus of the type that comprises a stem anchored in the humeral canal, which stem is provided on its upper or metaphyseal part with a bearing surface against which rests a hemispherical cap suitable to interact with the shoulder socket, characterized by the fact that the stem is chosen from among a set of stems, each of which has a bearing surface of different tilt angle with respect to the longitudinal axis of each stem.
With the invention, the surgeon is not limited to the direction he cuts, separating the articular portion from the osseous metaphysis. Thus, he can follow exactly the anatomy of the shoulder being operated on. According to the size of the patient, the surgeon then selects a set of several stems, e.g., 4 or 6. Then, the surgeon selects from this set of stems the stem for the prosthesis that, with respect to the tilt angle of its bearing surface, adapts best to the cut he made beforehand, in order to position with utmost precision the hemispherical cap on the frontal plane. The prosthesis in accordance with the present invention has the additional advantage that each stem may be manufactured in a single piece, in contrast to the adjustable prostheses that require several parts to be fitted together during an operation.
A first advantage of the present invention is that the bearing surface of each stem is provided with a round collar for the housing of a hemispheric cap.
The design can also be such that the round collar of each stem fits with reduced play into an eccentric housing on the base of the hemispheric cap. Such a structure allows a direct and easy fitting of the humeral stem on the spherical cap.
The present invention relates also to a set of humeral stems for a prosthesis of the upper extremity of the humerus, comprising several stems, each of which is provided with a bearing surface presenting a different tilt angle with respect to the longitudinal axis of each stem. Such a set of stems can be designed in several sizes that correspond to the possible different sizes of the patient; the diameter of the diaphysis can be between 6 and 15 mm.
According to an advantageous aspect of the set of stems of the present invention, a hemispheric cap is provided to interact with the bearing surface of each stem. This corresponds to the modular nature of the invention.
According to another advantageous aspect of the present invention, the hemispheric cap comprises a base provided with an eccentric housing into which fits, with reduced play, a circular collar that constitutes the bearing surface of each stem.
In this case, the circular collar of each stem, constituting the set of stems, can be designed in such a manner that it is tilted at a different angle with respect to the longitudinal axis of each stem.